A sensor arrangement of the kind described above is disclosed in U.S. Pat. No. 4,011,859. The sensor arrangement disclosed in this patent measures the CO.sub.2 -content in a respiratory gas. The respiratory gas flows through a measuring cuvette and an infrared beam is directed through this cuvette. The radiation originates from a pulsed infrared radiation source and is focused into a beam by a lens system and the rays of the beam are directed parallel to each other through the cuvette. The infrared radiation is focused on an infrared-sensitive detector after penetrating the cuvette. The infrared radiation is more or less attenuated in correspondence to the quantity of CO.sub.2 in the respiratory gas. To improve sensitivity, those wavelengths are filtered out of the infrared radiation to which the CO.sub.2 molecule responds especially characteristically. This filtering is performed with the aid of an interference filter. A wavelength of approximately 4.3 micrometers is conventionally selected for this purpose.
The respiratory gas utilized in the known arrangement as a sample gas contains substantial components of water vapor which deposits on the inner walls of the cuvette. However, even when detecting other compositions of sample gas, it cannot be assumed that the gas composition is free of water vapor. Accordingly, a reduction in the transmittance of the cuvette for the beam path must be accepted in these cases when water vapor or other dirt particles deposit on the inner walls of the cuvette and especially in the transmitting region for the radiation. To prevent a condensation of water vapor in the known arrangement, the entire cuvette holder is heated and maintained at a temperature preventing the condensation. The heated cuvette holder surrounds the cuvette except for those areas which must be kept free for the pass-through of the beam through the cuvette.
The known arrangement has the disadvantage that just that area through which the beam penetrates the cuvette and again passes therefrom, can only be heated indirectly. In lieu thereof, those parts are primarily heated which are inconsequential for the beam pass-through. This results in a large thermal mass which must be brought to a constant heating temperature and maintained there by means of a corresponding amount of heat energy. Furthermore, the heat must be transported from the heated cuvette zones via heat transport in the cuvette itself to the cuvette zones penetrated by the beam.
In addition, arrangements are disclosed in Austrian Patent 384,488 wherein electrical heating elements are applied directly to the cuvette or to the cuvette window. If the cuvette is to be removable for the purpose of cleaning and disinfection or sterilization as is required, then the electrical connection of the heat elements to the sensor is provided via plug contacts. This arrangement is disadvantageous because of the tendency to mechanically malfunction through repeated insertion of the contacts and the tendency of the electrical elements on the cuvette to corrode because of the cleaning or sterilization operations. Furthermore, it may be required for certain applications such as in the area of medicine to avoid external electrical contacts which can be touched.